The present invention relates to a multiaccess communications system and, more particularly, to such a multiaccess communication system as a carrier sensing multiaccess (CSMA) system or a collision detecting CSMA (CSMA/CD) system in a local communications network (LAN).
Local communications networks installed in private facilities, for example, include a so-called CSMA/CD system in which communications terminals are commonly connected to intermediate taps of a coaxial cable via individual transceivers. Each of the terminals is capable of transmitting an information signal to another terminal independently of the others. Upon collision of transmission between the terminals, the transmission is interrupted at each terminal to repeat it after a random waiting time. The coaxial cable may, for example, be 500 meters to 2 killometers long and provided with a transmission band of about 20 MHz so as to transmit base band pulse signals. Information which such a system may transmit includes picture, speech and data and is assembled in a packet or like message block. A destination address and such control codes as a check code are added to the message block.
The CSMA/CD system is used with ETHERNET or the like as a major communications system of a bus type local computer network (LCN). While the system exhibits relatively good performance under a simple control, the problem is that under a heavy load it encounters an increase in delay which eventually would lead to deterioration of the performance. Another problem is that considering the actual packet length L.sub.p the maximum allowable distance D between remotest stations is not more than about 2.5 killometers when the line speed C is 10 Mbps (megabits per second).
In light of the above, there has been proposed a channel division type CSMA/CD system which divides a channel capacity to provide a plurality of channels and effects random access channel by channel (presented by Yoshifumi Nomura in a paper entitled "Channel Division Type CSMA-CD System", Preparatory Manuscripts, Showa 58 (1983) National Conference of The Institute of Electronics And Communication Engineers of Japan. In accordance with the proposed system, the channel capacity is divided to set up a plurality of channels and, by allocating packet holding stations to the respective channels, access is made with the CSMA/CD approach. The proposed system, therefore, may offer a remarkable improvement in throughput-delay performance under a heavy load condition, compared to the prior art system which relies on a single large capacity channel.
Intuitively, division of a channel to n subchannels may be implemented in the base band by use of n cables. However, the increase in the number of cables by n times would render the construction intricate. The intricacy of construction would further be enhanced by the need for n transceivers which are to be connected to the n cables. In a wide band, while a single cable suffices if the frequency is divided into n frequencies, the intuitive technique employing the same principle as in the base band would require n transceivers for each of the taps. Should the number n be large, difficulty would be experienced in practicing the method for economic reasons.